Discoloration-resistant gold alloy

ABSTRACT

Alloy for the manufacturing of jewels or clock components with minimum concentrations of gold of 75 wt %, of copper between 5% and 21%, of silver between 0% and 21%, of iron between 0.5% and 4% and vanadium between 0.1% and 2.0%, intended to increase the tarnishing-resistance of alloys with a minimum content of gold of 75 wt % under environments in which sulphur- and chlorine-compounds are present.

CROSS RELATED APPLICATION

This application is the U.S. national phase of International ApplicationNo. PCT/IB2013/002683 filed 2 Dec. 2013 which designated the U.S. andclaims priority to Italian Patent Application No. RM2012A000608 filed 3Dec 2012, the entire contents these applications are incorporated byreference.

DESCRIPTION FIELD OF THE ART

The present invention relates to an alloy for the manufacturing ofjewels and/or clock components and/or the like with gold at a minimumconcentration of 75 wt %, copper at a concentration of between 5 wt %and 21 wt %, silver at a concentration of between 0 wt % and 21 wt %,iron at a concentration of between 0.5 wt % and 4 wt %, vanadium at aconcentration of between 0.1 wt % and 2.0 wt %, and iridium at aconcentration of between 0 wt % and 0.05 wt %. In a particularembodiment of the invention, the alloy comprises palladium in contentsranging from 0.5 wt % to 4 wt %.

STATE OF THE ART

Due to its high ductility, excellent thermal and electrical conductivityor high chemical inertness, gold has always been used in differentapplication fields and whenever these properties serve for majortechnological functions. Particularly, the unique optical and colorproperties of this element have been exploited since antiquity for themanufacturing of decorative objects.

Over the past few years, many gold alloys with defined functionalproperties have also been developed. Even today, many of the studiesfocusing on gold alloys are aimed at identifying particular, newchemical compositions which can meet the increasingly diverserequirements of clock-making industry or jewelry manufacturers. In fact,increasingly specific demands in the industrial field have madeindispensable the synthesis of compositions with innovative colorproperties. The color of a generic metal alloy is strictly dependent onits chemical composition because the mechanisms of interaction betweenthe incident light and the metal are a function of both the alloyingelements and the contents in which they are present within the alloy.For example, gold alloys with shades varying from green to yellow orrose (colored gold alloys) typically contain silver and copper, whereaselements such as palladium, platinum, nickel or manganese are added togold for the production of white alloys.

Due to recent developments in spectrophotometric techniques, the colorof a generic metal can be quantitatively and uniquely defined in thethree-dimensional domain CIE 1976 L*a*b* once the values of theCartesian coordinates L*, a* and b* are known (standard ISO 7224). Theparameter L* identifies the brightness and takes values ranging from 0(black) to 100 (white), whereas a* and b* are the chrominancecoordinates. Therefore, in this space, the achromatic grayscale isidentified by points on the axis L* where a*=b*=0, whereas a* and b*identify the color. Positive a* values denote red, negative a* valuesdenote green, positive b* values denote yellow, and negative b* valuesdenote blue. Furthermore, this color evaluation system can provide anestimate for the difference ΔE*(L*,a*,b*)=(ΔL*²+Δa*²+Δb*²)^(1/2) betweentwo different shades of color. ΔL*, Δa* and Δb* represent the arithmeticdifferences between the values of the coordinates L*,a*,b* identifyingthe two given shades in the space CIE 1976 L*a*b*. Generally, human eyeis able to distinguish between two different shades of color ifΔE*(L*,a*,b*)≥1.

Gold alloys may undergo unwanted surface discolorations over time as aresult of chemical/physical interactions which can occur between themetal and aggressive environments capable to promote phenomena ofcorrosion or tarnishing. According to the literature (“Tarnishresistance, corrosion and stress corrosion cracking of gold alloys”;Gold Bulletin, 29(2) pp 61-68, 1996; “Chemical stability of Gold dentalalloys”; Gold Bulletin, 17(2), pp 46-54, 1984), the phenomenon ofcorrosion is defined as a gradual chemical or electrochemical attackwhich can then result in a continuous dissolution of metal. Differently,the phenomenon of tarnishing is a specific form of corrosion. In thiscase, the reactions accompanying this phenomenon lead to the formationof thin layers of oxides, sulphides or chlorides which can alter thecolor and the surface gloss of gold alloys. These changes in surfacecolor properties can be quantified by evaluating the parameterΔE(L*,a*,b*) over time, as calculated with respect to the conditionsbefore the onset of corrosion phenomena.

18-carat gold alloys are traditionally considered not susceptible tocorrosion phenomena, thus being suitable for the manufacturing of jewelsor clock components. Indeed, recent studies and observations do not seemto confirm these considerations as they show that even high contents ofgold or other noble elements do not ensure an adequate chemicalstability over time under different conditions of use.

For example, a standard 18-carat alloy 5N ISO 8654 containing copper ina content of 20.5% and silver in a content of 4.5 wt % shows an apparentchemical instability even when subjected only to the action of a genericambient atmosphere. At a temperature of 25° C., the interactionsoccurring between the metal and the ambient atmosphere can alter thesurface color of the given gold alloy. These color changes are afunction of the time t of exposure to the aggressive action of theatmosphere environment, and they can be quantified byspectrophotometrically measuring the values of the coordinates L*, a*,b* on the surface of a sample of a 18-carat alloy 5N ISO 8654. Thevalues of the CIE 1976 coordinates L*a* b*as measured at defined timeintervals allow the kinetics of surface discoloration of the test sampleto be analyzed by evaluating the parameterΔE*(L*,a*,b*)=[(L*−L*₀)²+(a*−a*₀)²+(b*−b*₀ ²)]^(1/2) over time. Thisparameter is calculated with respect to the coordinates L*₀, a*₀, b*₀ ofthe test alloy as measured immediately after smoothing and subsequentpolishing of the surface of the test sample. This surface processing ofthe sample is performed until a constant reflection factor is achieved.Such a surface processing of the test sample is essential, and it iscarried out in order to remove traces of any compound (e.g. oxides)which can alter the surface composition of the alloy and its actualcolor, thereby having the potential to distort the experimentalmeasurements. The results of these tests allow obtaining experimentalcurves ΔE*(L*,a*,b*) vs. time, as shown in FIG. 1. The curve shownherein can then be analyzed. Since time t=0 corresponds to theconditions immediately after polishing, then the value of ΔE*(L*,a*,b*,t=0) is zero. The value of this parameter tends to vary widely duringthe early days of the test. In fact, after about 5 days from the startof the test, the material undergoes a perceptible color changeΔE*(L*,a*,b*)≥1. Beyond this time interval, the value of the parameterΔE*(L*,a*,b*) continues to increase but the rate at which the colorvaries over time decreases, until the parameter ΔE*(L*,a*,b*) almostasymptotically reaches a plateau of values of less than 2.5.

The manner in which corrosion phenomena occur in gold alloys is closelyrelated to the composition of alloys. With increased levels of silver,copper or other elements capable of degrading the typical chemicalstability of gold, the chances of initiating corrosion phenomena ofdifferent nature increase. Similarly, the kinetics of the chemical orelectrochemical reactions accompanying the alteration in surfaceproperties of the manufactured articles will be also favored.

The manner in which the tarnishing or corrosion phenomena occur may alsobe related to microstructural features of gold alloys. From ametallurgical point of view, any microstructural inhomogeneity cangenerate differences in electrical potential within the material,thereby decreasing its chemical stability. For this reason, homogeneoussolid solutions generally have an increased chemical stability againstcorrosion compared to alloys whose microstructures are formed by eithermultiple immiscible phases or different structural components. Inaddition, grain boundaries may constitute preferential sites ofinitiation for corrosion phenomena. The size of the crystal grain(standard ISO 643) influences the chemical stability of a gold alloybecause the average size of crystal grains is inversely proportional tothe energy of grain boundary. This energy, which is defined as the freeenergy of the polycrystalline structure in excess to that of the perfectlattice, can cause a decrease in chemical stability of the alloy,thereby increasing the electrochemical potential differences establishedbetween either the alloying elements or the segregated phases.Eventually, the presence of any residual stress generated by the volumeshrinkage of the material during solidification or cold plasticdeformation processing, can give rise to phenomena of stress corrosionand lead to undesired fractures in the material.

The environments capable of promoting corrosion in gold alloys aremultiple, and they are related to the applications of the alloys. In thejewelry and clock-making industries, colored alloys containing silver orcopper appear to be particularly susceptible to tarnishing phenomena.Both chloride-containing solutions, such as seawater, andsurfactant-containing solutions can initiate undesired changes insurface color of this type of gold alloys within a short time.Similarly, moisture, organic vapors, oxygen compounds and especiallysulphur compounds, such as hydrogen sulphide H₂S, existing in theenvironmental atmosphere, are also able to initiate tarnishingphenomena. Eventually, the same problems may arise from the interactionwith organic solutions such as sweat, in which salts such as sodiumchloride, electrolytes, fatty acids, uric acid, ammonia and urea areprimarily dissolved.

Therefore, colored gold alloys, which are characterized by shadesranging from green to yellow or rose and which are typically employedfor the manufacturing of jewels or clock components, can distinctivelyshow an inadequate chemical stability and undergo unwanted changes insurface color properties over time. The present invention seeks toimprove the chemical stability of currently commercially availablecolored gold alloys. Particularly, the aim is to increase thetarnishing-resistance of alloys containing gold in a minimum content of75 wt % under environments in which sulphur- or chlorine-compounds arepresent.

Technical literature discloses several chemical compositions in whichelements such as germanium, indium, cobalt, gallium, manganese, zinc,tin or iron are added to the basic ternary gold-silver-copper system inorder to obtain particular physical or functional properties. Thecompositions shown below are all expressed as percentages by weight.

Document JP2008179890A (2008) considers germanium as an element whichcan increase the corrosion-resistance of 18-carat gold alloys.Particularly, compositions with contents of germanium in a range from0.01% to 10% are envisaged.

Document JP2002105558A (2002) also discloses concentrations of germaniumin a range from 3% to 5% in compositions characterized by at least 75%of gold, contents of copper between 12% and 13%, and silver to balance.In this case, germanium is not considered to improve the chemicalstability of 18-carat rose alloys, but only to achieve desired colorproperties.

Document CA2670604A1 (2011) discloses compositions comprising gold in acontent between 33.3% and 83%, indium in a content between 0.67% and4.67%, tin in a content up to 0.9%, manganese in a content up to 0.42%,silicon in a content up to 0.04%, and copper to balance. In this case,indium is used to obtain gold alloys with colors similar to those ofbronzes.

On the other hand, document U.S. Pat. No. 7,413,505 (2008) proposes14-carat rose gold alloys in which, in addition to copper, silver andzinc, cobalt in contents between 3% and 4% is added to the alloy inorder to achieve specific values of hardness. The same documentdiscloses similar 18-carat alloys whose compositions are, however, notclaimed.

In order to obtain improved hardness and corrosion-resistance comparedto those of standard alloys employed in dentistry, documentJP2009228088A (2009) proposes the addition of gallium in a range between0.5% and 6% to gold alloys characterized by comprising gold in a contentgreater than 75%, platinum in a content between 0.5% and 6%, palladiumin a content between 0.5% and 6%, and copper to balance.

Instead, document JP2001335861 (2001) claims the addition of manganesein contents between 2% and 10% to alloys comprising gold in a minimumcontent of 75%, copper in a content between 10% and 30%, silver in acontent between 0.5% and 3%, zinc in a content between 0.5% and 3%, andindium in a content between 0.2 and 2%.

Eventually, document GB227966A (1985) discloses alloys comprising goldin a content between 33% and 90%, iron in a content between 0.1% and2.5%, silver in a content between 0.01% and 62.5%, copper in a contentbetween 0.01% and 62.5%, zinc in a content between 0.01% and 25%, andcharacterized by hardness values in a range from 100 HV to 280 HV.

Still further, document JP2008308757 (2008) considers the addition of0.5%-5% of tin to gold alloys containing copper in a content between14.5% and 36.5% and indium in a content between 0.5% and 6%. In thiscase, the invention only claims that rose gold alloys can be obtainedwhile avoiding the use of elements such as nickel, manganese andpalladium and the disadvantages resulting from their use. In fact, as itis known, nickel can cause allergies, manganese in addition to decreasecold plastic deformation processability, requires require the use ofadvanced manufacturing technologies, and palladium reduces surfacebrightness.

As previously stated, palladium is an element which is typically addedto gold for the synthesis of white alloys. Certain documents report theuse of this chemical element also in colored gold alloys because, evenif it generates dark, low-glossy surfaces, it can effectively increasethe resistance against corrosion phenomena.

In fact, even palladium contents of less than 3 wt % (“Effect ofpalladium addition on the tarnishing of dental gold alloys”; J. Mater.Sci.-Mater., 1(3), pp. 140-145, 1990; “Effect of palladium on sulfidetarnishing of noble metal alloys”; J. Biomed. Mater. Res., 19(8), pp.317-934, 1985) minimize the tarnishing effects generated by environmentsin which sulphur compounds are especially, present. In this case,palladium can reduce the growth of the surface layer mainly consistingof silver sulphide (Ag₂S). Contrary to what happens with silver, asurface enrichment of palladium doesn't occur, but it is possible toobserve a statistical increase in the content of such an element in thelayer immediately below the outermost layer of sulphides. This localizedincrease in palladium reduces the diffusion of S²⁻ ions from the surfaceregion to the core of the manufactured articles, thereby consequentlydecreasing the growth of the layer of sulphides and the change insurface color of the gold alloys containing it.

For example, document JP60258435A (1985) considers palladium as anelement capable of improving the chemical stability of 18-carat goldalloys characterized by comprising copper in a content between 15% and30% and silver in a content between 5% and 25%. In this case, theinvention discloses additions of palladium in a range from 4% to 7%.

Document JP10245646A (1998) also proposes additions of palladium in arange between 0.3% and 5% to rose gold alloys (L*=86÷87, a*=, 8÷10 a*and b*=17÷22) comprising gold in a content between 75% and 75.3%, copperin a content between 15% and 23%, and silver to balance. This inventiondoes not consider palladium as an element capable to increase theresistance against corrosion phenomena, but discloses its use toincrease the castability and toughness of the material.

Eventually, document EP1512765A1 (2005) also discloses additions ofpalladium in amounts of less than 4%, among the various claims.Furthermore, for the same purpose, it also envisages additions ofplatinum in amounts between 0.5% and 4% to alloys which comprise gold ina content higher than 75% and copper in a content between 6% and 22%,and in which minimal additions of silver, cadmium, chromium, cobalt,iron, indium, manganese, nickel or zinc may be present in an amount ofless than 0.5%. These compositions have been developed for the synthesisof rose gold alloys with a high resistance against surface color changeunder environments in which chlorine compounds may be present.

Several documents (WO2009092920, DE3211703, EP2251444, DE102004050594,DE10027605A1, EP0381994, U.S. Pat. No. 4,820,487) disclose additions ofvanadium and other elements such as iron, chromium, zirconium, hafnium,titanium or tantalum to white gold alloys. However, in the documentscited above, such additions are considered only to improve themechanical features of the claimed compositions or to achieve particularcolor properties.

DESCRIPTION OF THE INVENTION

The present invention seeks to improve the chemical stability ofcurrently commercially available colored gold alloys. The aim is toincrease the tarnishing-resistance of alloys with a minimum content ofgold of 75 wt % under environments in which sulphur- orchlorine-compounds are present.

Particularly, the present invention seeks to increase the chemicalstability of high-carat colored alloys by providing for the addition ofiron and vanadium to the basic gold-silver-copper system. Particularly,the invention discloses alloy compositions containing gold at aconcentration higher than 75 wt %, copper at a concentration between 5%and 21%, silver at a concentration between 5% and 21%, iron at aconcentration between 0.5% and 4%, and vanadium at a concentrationbetween 0.1% and 2%.

DESCRIPTION OF TABLES AND FIGURES

TABLE 1 shows the composition and the main physical characteristics ofthe alloys disclosed in the present document. For each composition, thevalues tabulated in columns L*₀, a*₀, b*₀ are evaluated with the use ofa spectrophotometer Konica Minolta CM-3610d. These measurements areperformed under reflection conditions with the use of a light sourceD65-6504K, a di/de observation angle of 8°, and a measurement area of 8mm (MAV). The measurements are carried out on samples immediately aftera careful processing of their surfaces. The surface processing ofsamples of the various compositions disclosed herein includes smoothingwith abrasive papers followed by polishing. Smoothing is performed bymeans of abrasive papers, whereas polishing is carried out with diamondpastes having a grain size of up to 1 μm. This processing is carried outuntil a constant reflection factor is reached. Such a processing isessential, and it is carried out in order to remove traces of anycompound which can alter the surface composition of the alloy and itsactual color, thereby having the potential to distort the experimentalmeasurements. The hardness values shown herein are measured after aflatbed lamination hardening of the material to 70% (column “70%hardened”), after an annealing treatment at 680° C. (column “Annealed”),and after a heat-treatment hardening performed at a temperature of 300°C. (column “Aged”). Hardness tests are carried out with an applied loadof 9.8N (HV1) which is maintained for 15 seconds, as specified bystandard ISO 6507-1.

Table 2 shows the ΔE(L*,a*,b*) values measured after 150 hours ofexposure to thioacetamide vapors (column “Exposure to thioacetamidevapors (150 hrs)”) and after 175 hours of immersion in a saturatedsolution of sodium chloride at neutral pH and at a thermostatedtemperature of 35° C. (column “Immersion in saturated aqueous NaCl (175hrs)”). The values shown for parameters ΔE(L*,a*,b*) relate tospectrophotometric measurements of the values of coordinates L*,a*,b* astaken at defined time intervals. The values thus obtained forcoordinates CIE 1976 L*a*b* allow the kinetics of surface discolorationof the test sample to be quantified by evaluating the parameterΔE*(L*,a*,b*)=[(L*−L*₀)²+(a*−a*₀)²+(b*−b*₀ ²)]^(1/2) over time. Thisparameter is calculated with respect to the values of coordinates L*₀,a*₀, b*₀ for the test alloy (values shown in table 1).

FIG. 1 shows the change in surface color for an alloy 5N ISO 8654 whileexposed to a generic ambient atmosphere at 25° C.

FIG. 2 shows the color changes ΔE(L*,a*,b*) for composition 5N ISO 8654,composition L11 and composition L01 as evaluated while carrying outtests according to standard ISO 4538.

FIG. 3 shows the color changes ΔE(L*,a*,b*) for compositions L01, L02,L03 and L04 as evaluated while carrying out tests according to standardISO 4538.

FIG. 4 shows the color changes ΔE(L*,a*,b*) for compositions 3N ISO 8654and L05 as evaluated while carrying out tests according to standard ISO4538.

FIG. 5 shows the color changes ΔE(L*,a*,b*) for composition 5N ISO 8654,composition L11 and composition L01 as evaluated while carrying outtests by immersing the various samples in a saturated solution of sodiumchloride NaCl at neutral pH and at a thermostated temperature of 35° C.

FIG. 6 shows color changes ΔE(L*,a*,b*) for compositions L01, L03 andL06 as evaluated while carrying out tests by immersing the varioussamples in a saturated solution of sodium chloride NaCl at neutral pHand at a thermostated temperature of 35° C.

FIG. 7 shows the color changes ΔE(L*,a*,b*) for compositions L01, L03and L06 as evaluated while carrying out tests according to standard ISO4538.

FIG. 8 shows the micro-structure of an alloy comprising iron in acontent of 1.8 wt%, vanadium in a content of 0.4 wt%, and iridium in acontent of 0.01 wt%.

DETAILED DESCRIPTION OF THE INVENTION

The different compositions disclosed in the present invention are meltedby using an induction furnace equipped with a graphite crucible, andthey are melted in graphite molding boxes of rectangular section. Thehomogeneity of the bath during melting is ensured by electromagneticinduction stirring. The pure elements (Au 99.999%, Cu 99.999%, Pd99.95%, Fe 99.99%, Ag 99.99%, V≥99.5%) are melted and cast under acontrolled atmosphere. Particularly, melting operations are carried outonly after at least 3 cycles of conditioning of the atmosphere of themelting chamber. This conditioning includes reaching a vacuum level upto pressures below 1×10⁻² mbar, followed by partially saturating theatmosphere with argon to 500 mbars. During melting, argon pressure ismaintained at pressure levels in a range from 500 mbars to 800 mbars.When pure elements are completely melted; the liquid is overheated up toa temperature of about 1250° C. in order to homogenize the chemicalcomposition of metal bath. During overheating, a vacuum level of lessthan 1×10⁻² mbar is reached again, which is useful to eliminate aportion of the slag produced while the pure elements are being melted.At this point, the melting chamber is partially re-pressurized to 800mbars with argon, and then the molten material is poured into thegraphite molding box. Once solidification has occurred, the resultingmelts are extracted from the molding box, quenched in water to preventphase changes to solid state, and then plastically cold-deformed byflatbed lamination.

During the cold plastic processing process, the different compositionssynthesized according to the melting procedure described above aredeformed up to 70%, then subjected to a heat annealing treatment attemperatures above 680° C., and subsequently quenched in water toprevent a phase change to solid state. During the entire process, allthe compositions shown herein are subjected to hardness testing in thehardened and annealed state. Additional hardness measurements are madeafter a heat-treatment hardening carried out at a temperature of 300° C.Hardness tests are performed with an applied load of 9.8N (HV1) which ismaintained for 15 seconds, as specified by standard ISO 6507-1.

Samples are taken from the materials processed by the processingprocedures described above, i.e. after melting, lamination,heat-treatment annealing and subsequent quenching, for metallographicanalysis. These samples are smoothed, polished and analyzed in order toevaluate the microstructural properties of the synthesized compositions.Similarly, additional samples of material are taken from the materialsprocessed by the processing procedures described above, and they aresubjected to color measurements and accelerated corrosion testing.

The surface of the samples subjected to color measurements andaccelerated corrosion testing are carefully smoothed by means ofabrasive papers and subsequently polished with diamond pastes with agrain size of up to 1 μm,until the achievement of a constant reflectionfactor. Such a surface processing of the samples is essential, and it iscarried out in order to remove traces of any compound which can alterthe surface composition of the alloy and its actual color, therebydistorting the experimental measurements.

Color measurements were made using a spectrophotometer Konica MinoltaCM-3610d immediately after the preparation of the samples and during thevarious corrosion tests. These measurements are carried out underreflection conditions with the use of a light source D65-6504K, a di/deobservation angle of 8°, and a measurement area of 8 mm (MAV).

The resistance to surface color change of the different compositionsproposed herein is evaluated in accordance with the test proceduresprescribed by standard ISO 4538. This standard establishes apparatus andprocedure for evaluating the corrosion- and oxidation-resistance ofmetal surfaces under an atmosphere containing volatile sulphides. Tothis aim, the specimens are exposed to thioacetamide vapors CH₃CSNH₂under an atmosphere having a relative humidity of 75% which ismaintained with the use of a saturated solution of sodium acetatetrihydrate CH₃COONa.3H₂O.

Furthermore, in order to evaluate the resistance to surface color changeunder environments characterized by the presence of chlorides, furthertests are carried out by immersing the samples in a saturated solutionof NaCl at neutral pH and at a thermostated temperature of 35° C.

Color changes occurring in the compositions analyzed by acceleratedcorrosion testing are a function of the time t of exposure to theaggressive action of test environments. Such changes can be evaluatedexperimentally by taking spectrophotometric measurements of coordinatevalues L*,a*,b* from the surface of the test alloy samples at definedtime intervals. The values thus obtained for coordinates CIE 1976 L*a*b*allow the kinetics of surface discoloration of the test material to bequantified by evaluating the parameterΔE*(L*,a*,b*)=[(L*−L*₀)²+(a*−a*₀)²+(b*−b*₀ ²)]^(1/2) over time. Thisparameter must be evaluated with respect to coordinates L*₀, a*₀, b*₀ ofthe test material as measured immediately after smoothing with abrasivepapers and subsequent polishing with diamond pastes with a grain size ofup to 1 μm. These operations are carried out until a steady reflectionfactor is reached. Such a surface processing of the sample is essential,and it is carried out in order to remove traces of any compound whichcan alter the surface composition of the alloy and its actual color,thereby having the potential to distort the experimental measurements.The results of these tests allow experimental curves ΔE*(L*,a*,b*) vs.time to be obtained, which are indispensable to analyze the kinetics ofcolor change in the analyzed compositions and, therefore, toquantitatively analyze the chemical stability in considered testenvironments.

Compositions and main physical characteristics of the alloys consideredin the present document are shown in table 1. On the contrary, table 2shows the values of ΔE(L*,a*,b*) as measured after 150 hours of exposureof the analyzed compositions to thioacetamide vapors, and after 175hours of immersion of the analyzed compositions in the solutioncontaining sodium chloride.

Additions of iron and vanadium of more than 1% and 0.1 wt %respectively, allow surface color change to be decreased under anatmosphere containing volatile sulphides. In this way, it is notrequired to add palladium in order to improve the chemical stability ofthe analyzed compositions, thereby avoiding the decrease of surfacebrightness due to the presence of this element within the alloy.Similarly, expensive additions of platinum are not required.

The curves shown in FIG. 2 can then be analyzed. Since time t=0corresponds to conditions immediately after the polishing of the samples5N ISO8654, L11, L01, then the value of ΔE*(L*,a*,b*, t=0) for the threedifferent given compositions is zero. As can be seen, after 150 hours ofexposure to thioacetamide vapors, for an alloy containing iron in acontent of 1.8 wt % and vanadium in a content of 0.4 wt % (L01), colorchange ΔE(L*,a*,b*) is 2.9. Under the same conditions, an alloy 5N ISO8654 undergoes a change of 5.6, whereas such a parameter for an alloy(L11) according to document EP1512765A1 has a value of 4.1.

Furthermore, for alloys having a composition falling within thisembodiment of the invention, the kinetics of discoloration occurringduring testing differs from those of the two compositions taken as areference. As can be also seen in FIG. 2, with reference to the alloy 5NISO 8654, a rapid color change occurs within the first 24 hours of thetest. Subsequently, the kinetics of color change decreases, but theparameter ΔE(L*,a*,b*) continues to increase throughout the 150 hours oftesting analyzed. The alloy L11 also shows a similar behavior, but afterabout 120 hours of exposure to thioacetamide vapors, the values ofparameters ΔE(L*,a*,b*) for this composition reach a plateau of almostconstant values. On the contrary, color change for composition L01 isstabilized after only 80 hours of testing.

Again, the presence of iron in the composition of the alloy allows themiscibility of vanadium in gold to be increased. Keeping a ratio greaterthan 4 between of iron and vanadium levels, allows obtaining solidsolutions and preventing second phases from separating out from themixture.

The curves shown in FIG. 3 can then be analyzed. Since time t=0corresponds to the conditions immediately after the polishing of thesamples L01, L02, L03, L04, then the value of ΔE*(L*,a*,b*, t=0) for thefour different given compositions is zero.<Compositions in whichpalladium is replaced with iron show a decreased resistance to colorchange under environments characterized by the presence of volatilesulphides. After 150 hours of exposure to the thioacetamide vapors, analloy with 1.8 wt % of palladium and 0.4 wt % of vanadium (L03)undergoes a change ΔE(L*,a*,b*) of 4.1, thus showing a surface colorchange which is comparable to that of the composition L11. However inthis case, (FIG. 3), is not possible to observe a stabilization of theparameter ΔE(L*,a*,b*) for the composition L03 within the first 150hours of testing.

Moreover, the addition of vanadium is essential to increase the chemicalstability of considered compositions. Under atmospheres containingvolatile sulphides, a simple addition of 1.8 wt % of iron (L02) resultsin a color change which is completely equivalent to that shown by thereference alloy 5N ISO 8654 (FIG. 3).

If palladium is substituted for iron, the effects generated by thepresence of vanadium are less obvious. As also shown in FIG. 3, after150 hours of exposure to thioacetamide vapors, a composition onlycharacterized by palladium in a content of 1.8 wt % (L04) undergoes acolor change ΔE(L*,a*,b*) of 3.8. For a composition in which vanadium isalso present, this parameter has a value of 4.1. In this case, thepresence of vanadium does not affect the chemical stability ofquaternary gold-silver-copper-palladium system. Furthermore, thecompositions L03 and L04 are not only characterized by the same chemicalstability, but also by the same kinetics of color development throughoutthe entire test range.

In case in which palladium is present in the alloy in substitution foriron, the effect of vanadium becomes appreciable only after the contentof silver is increased and the content of copper is decreased. This isthe case of an alloy comprising silver in contents between 5% and 16 wt%, palladium in contents between 0.2% and 5 wt %, and vanadium incontents between 0.2% and 1.5 wt %. The curves shown in FIG. 4 can thenbe analyzed. Since time t=0 corresponds to the conditions immediatelyafter the polishing of the samples 3N ISO8654, L05, then the value ofΔE*(L*,a*,b*, t=0) for the two different given compositions is zero. Forexample (FIG. 4), after 150 hours of exposure to the thioacetamidevapors, an alloy comprising silver and copper in contents of 12.5% byweight and additions of palladium and vanadium of 1.8% and 0.4 wt %respectively (L05) shows a color change ΔE(L*,a*,b*) of 3.6. Under thesame conditions, a standard alloy 3N ISO 8654 undergoes a change of 4.8.In this particular embodiment of the invention, the additions ofpalladium allow the miscibility of vanadium in gold to be increased.

Tests performed by immersing the samples into the solution of sodiumchloride (FIG. 5) confirm the chemical stability of the alloy L11disclosed in document EP 1512765A1. After 175 hours of immersion in thechloride-containing solution, such a composition undergoes a colorchange ΔE(L*,a*,b*) of 1.9, while such a parameter for a composition 5NISO 8654 has a value of 3.6. Under the same conditions, the compositionL01 undergoes a change of 2.7. Accordingly, simple additions of iron orvanadium cannot optimize the strength of gold alloys in solutions inwhich chlorides are dissolved.

To this aim, a further embodiment of the invention provides foradditions of palladium in a range from 0.5% to 2 wt %, iron in a rangefrom 0.5% to 2 wt %, and vanadium in a range from 0.1% to 1.5 wt %.

After 175 hours of immersion in the chloride-containing solution, analloy characterized by 0.9 wt % of iron, 0.9 wt % of palladium and 0.4wt % of vanadium (L06) undergoes a color change ΔE(L*,a*,b*) of 2.1. Thecurves shown in FIG. 6 can then be analyzed. Since time t=0 correspondsto the conditions immediately after the polishing of the samples L01,L03, L06, then the value of ΔE*(L*,a*,b*, t=0) for the three differentgiven compositions is zero. As can be seen in FIG. 6, the color changeof the alloy L11 is quick within the first 48 hours of testing and afterabout 150 hours of immersion, and the values of the parameterΔE(L*,a*,b*) reach a plateau of almost constant values. On the contrary,the composition L06 undergoes a rapid color change within the first 24hours, and similarly to what happens with the composition L11, theparameter ΔE(L*,a*,b*) of the composition L06 is also stabilized afterabout 150 hours of testing.

This further embodiment of the invention allows the resistance to colorchange to be increased in solutions in which chlorides are dissolved.However, at the same time, the chemical stability under environmentscontaining volatile sulphides is maintained. The curves shown in FIG. 7can then be analyzed. Since time t=0 corresponds to the conditionsimmediately after the polishing of the samples L01, L03, L06, then thevalue of ΔE*(L*,a*,b*, t=0) for the three different given compositionsis zero. As shown in FIG. 7, after 150 hours of exposure tothioacetamide vapors, the composition L06 undergoes a color changeΔE(L*,a*,b*) of 3.3. This color change reaches a plateau of intermediatevalues compared to those of the compositions L01 and L03.

Furthermore, compositions in which the ratio of the sum of theconcentrations of iron and palladium to the concentration of vanadium isgreater than 4, are solid solutions which are homogeneous and free ofsecond phases.

By replacing palladium with iron, it is possible to obtain an increasedsurface brightness. As shown in table 1, the composition L01 ischaracterized by a parameter L* of 86.66, whereas such a parameter forthe composition L04 has values lower than and equal to 85.21. The L*values obtained by partially replacing palladium with iron, as in thecase of the composition L06, are intermediate values compared to thoseset forth above.

Iron and vanadium are chemical elements capable to decrease the shadesaturation of gold alloys. The higher the concentration of theseelements, the lower the values of coordinates a* and b* and the more thecolors will become achromatic.

To overcome this problem, a further embodiment of the inventiondiscloses compositions in which silver may not be present and whichcomprise copper in a content between 16% and 23 wt %, iron in a contentbetween 0.5% and 4 wt %, and vanadium in a content between 0.1% and 1 wt%. For example, with the composition L07 in which iron is present at aconcentration of 2.5 wt % and the content of vanadium is 0.6 wt %, it ispossible to obtain an a* value of 6.45 which is similar to that reportedfor the composition L01. However, the absence of silver causes adecrease in parameter b* (yellow). In fact, the composition L07 ischaracterized by a b* value of 12.90, whereas this parameter takes avalue of 15.49 for the composition L01. Also with this particularembodiment of the invention, which includes compositions in which theratio between the concentrations of iron and vanadium is more than 4,solid solutions are obtained which are homogeneous and free of secondphases.

Moreover, the presence of iron causes an increase in surface brightness.An alloy with 2.5 wt % of palladium (L09) is characterized by an L*value of 83.77. The composition L07 in which iron is present in acontent of 2.5 wt % is characterized by an L* value of 86.09. When ironcontent is increased to 3.1 wt %, even in the absence of vanadium (L08),the parameter L* takes a value of 86.33.

A last embodiment of the invention may comprise iridium in contents ofless than 0.05 wt %. These additions allow the crystal structure of thecompositions considered to be tuned. FIG. 8 shows the micro-structure ofan alloy comprising iron in a content of 1.8 wt %, vanadium in a contentof 0.4 wt %, and iridium in a content of 0.01 wt %, which has beenplastically cold-deformed up to 70% and annealed at 680° C. Thecomposition is characterized by a grain size of 7 according to standardISO 643. A similar grain size allows the manufactured articles to show agood polishing ability. Increased additions of iridium can furtherincrease the grain size index and have adverse effects on the chemicalstability of the alloy.

TABLE 1 Color CIE Hardness HV1 L*a*b* 70% Alloy Composition [wt %] L*0a*0 b*0 Hardened Annealed Aged L01 Au75 Ag4.1 Cu18.7 Fe1.8 V0.4 86.866.45 15.49 267 170 265 L02 Au75 Ag4.2 Cu19.0 Fe1.8 86.88 6.47 15.50 261162 273 L03 Au75 Ag4.1 Cu18.7 Pd1.8 V0.4 85.54 7.32 14.17 256 160 285L04 Au75 Ag4.2 Cu19.0 Pd18 85.21 8.23 14.47 254 156 298 L05 Au75 Ag11.4Cu11.4 Pd1.8 V0.4 87.27 5.16 17.30 239 154 215 L06 Au75 Ag3.6 Cu19.2Pd0.9 Fe0.9 V0.4 85.77 6.85 14.10 273 165 275 L07 Au75 Cu21.9 Fe2.5 V0.686.09 6.45 12.90 295 192 323 L08 Au75 Cu21.9 Fe3.1 86.33 5.78 12.75 272163 302 L09 Au75 Cu22.5 Pd25 83.77 8.11 11.74 245 163 286 L10 Au75 Ag4.1Cu18.7 Fe1.8 V0.4 Ir0.01 86.80 6.43 15.49 265 172 260 L11 Au76 Pt3 Cu2184.52 9.10 13.10 270 165 300 5N ISO Au75 Ag4.5 Cu20.5 86.94 9.60 17.50230 165 325 8654 3N ISO Au75 Ag12.5 Cu12.5 89.30 5.68 22.45 220 145 2308654

TABLE 2 ΔE(L*, a*, b*) Exposure to Immersion in a thioacetamidesaturated aqueous vapors solution of NaCl Alloy Composition [wt %] (150hours) (175 hours) L01 Au75 Ag4.1 Cu18.7 Fe1.8 2.9 2.7 V0.4 L02 Au75Ag4.2 Cu19.0 Fe1.8 4.7 2.9 L03 Au75 Ag4.1 Cu18.7 Pd1.8 4.1 1.8 V0.4 L04Au75 Ag4.2 Cu19.0 Pd18 3.3 2.4 L05 Au75 Ag11.4 Cu11.4 Pd1.8 3.6 2.0 V0.4L06 Au75 Ag3.6 Cu19.2 Pd0.9 3.3 2.1 Fe0.9 V0.4 L07 Au75 Cu21.9 Fe2.5V0.6 4.2 2.6 L08 Au75 Cu21.9 Fe3.1 4.4 3.0 L09 Au75 Cu22.5 Pd25 4.7 2.0L11 Au76 Pt3 Cu21 4.1 1.9 5N ISO Au75 Ag4.5 Cu20.5 5.6 3.6 8654 3N ISOAu75 Ag12.5 Cu12.5 4.8 3.3 8654

The invention claimed is:
 1. A tarnish resistant gold alloy formanufacturing jewels or clock components, consisting of: gold at aconcentration of 75 percent by weight (wt %), copper at a concentrationof 18.7 wt %, silver at a concentration of 4.1 wt %, iron at aconcentration of 1.8 wt %, and vanadium at a concentration of 0.4 wt %.2. A tarnish resistant gold alloy for manufacturing jewels or clockcomponents, consisting of: gold at a concentration of 75 percent byweight (wt %), copper at a concentration of 19.2 wt %, silver at aconcentration of 3.6 wt %, palladium at a concentration of 0.9 wt %,iron at a concentration of 0.9 wt %, and vanadium at a concentration of0.4 wt %.
 3. A tarnish resistant gold alloy for manufacturing jewels orclock components, consisting of: gold at a concentration of 75 percentby weight (wt %), copper at a concentration of 19 wt %, silver at aconcentration of 4.2 wt %, and palladium at a concentration of 1.8 wt %.